We propose to investigate the interfacial electron transfer reaction of several proteins in the absence and presence of redox mediators. The purpose is to gain an understanding of the energetics and kinetics of the direct and mediated electron transfer reaction of proteins at electrode surfaces. To accomplish this, a voltammetric study will be carried out to characterize the systems under study. Spectroelectrochemistry and rotating disk technique will be employed to determine thermodynamic and kinetic parameters, respectively. The proteins chosen for study were selected because of their ability to catalize the oxidation of substrates of physiological and/or pathological significance. These enzymatic reactions may represent the basis for the future development of voltammetric sensors that may find application in the clinical laboratory for the diagnosis of diseases and/or disorders caused by abnormal concentrations of metabolites.